This invention relates to devices and methods for chemical analysis of materials. Specifically this invention relates to headspace analysis, and apparatus and methods used to increase sensitivity and repeatability in conducting headspace analysis.
Headspace analysis is a relatively new technique which enables the sampling of a vapor phase of a sample with an analytical instrument. Headspace sampling ensures that only volatile materials are delivered for analysis. For example when the analytical instrument used is a gas chromatograph, headspace sampling assures that only volatile species that can be eluted from a column of a gas chromatograph will be introduced into the instrument.
In headspace sampling a volatile non-vapor phase of a substance being analyzed, which may be either liquid or solid attains equilibrium with a vapor phase of the substance within a sealed vial. Equilibrium is established when the non-vapor phase of the substance in the vial no longer changes so that the total quantity of the vapor and non-vapor phases remains constant. Often a syringe is used to retrieve a small sample of the vapor for analysis. The retrieved vapor is then introduced into an analytical instrument. Headspace technology is advantageous over conventional direct sample injection techniques because it allows only vapor to enter the analytical instrument. This is advantageous because it reduces the chance of contamination or damage to the analytical instrument due to introduction of unevaporated sample material. Because the sample is in vapor form, greater sample volumes may be supplied to the instrument. Increased sample size generally results in increased sensitivity.
Samples of headspace vapor may be extracted from a sample vial using a number of other techniques. Such techniques often involve equilibrating the vapor and non-vapor phase of a substance for analysis within a closed vial. A sample needle is moved to pierce a septum bounding the headspace in the vial. As a result a fluid passage through the needle is in fluid communication with the vapor phase of the sample in the headspace. To extract the headspace sample it is usually necessary to first pressurize the headspace with a suitable gas.
After the headspace has been pressurized the pressure is released allowing the sample material to pass out of the vial and into an analytical instrument or other device for collecting or analyzing the sample. Techniques for extracting headspace vapor from a vial are shown in U.S. Pat. No. 5,441,700 the disclosure of which is incorporated by reference as if fully rewritten herein.
A drawback associated with conventional techniques for the extraction of sample vapor from a headspace vial is that variations in pressure must be achieved to extract the sample material. Such variations in pressure often change the equilibrium conditions between the vapor phase and the non-vapor phase of the substance being analyzed. Changes in equilibrium may change the makeup of the headspace vapor. Such changes which result from the sampling process often impact the results in ways that are undesirable.
Thus there exists a need for a headspace sampling apparatus and method which minimizes the effects of the sampling process on the constituents in the sample and which increases sample volumes which may be delivered and/or analyzed by an analytical instrument.
It is an object of the present invention to provide a sampling apparatus which achieves the sampling of headspace vapors while minimizing the disturbance of thermodynamic equilibrium.
It is a further object of the present invention to provide a sampling apparatus with improved sensitivity.
It is a further object of the present invention to provide a sampling apparatus which increases sample volumes which may be analyzed by an analytical instrument.
It is a further object of the present invention to provide an improved method of sampling materials for analysis.
It is a further object of the present invention to provide a method of sampling that reduces the effects of pressure changes on a headspace sample.
It is a further object of the present invention to provide a method of sampling that may achieve greater sample size.
It is a further object of the present invention to provide a method of sampling which achieves increased sensitivity.
It is a further object of the present invention to provide a method of sampling under conditions of thermodynamic equilibrium.
Further objects of the present invention will be made apparent in the following Best Modes for Carrying Out Invention and the appended claims.
The foregoing objects are accomplished in an exemplary embodiment of the invention by a system and method in which a material to be analyzed is held in a generally sealed vial. The vial is positioned in an equilibration chamber. At least two needles or similar fluid passages extend into the otherwise sealed vial. The needles extend into the headspace which holds a vapor phase of the material being analyzed which is generally above a non-vapor phase of the material.
One of the needles is in fluid communication with a first variable volume chamber. The second needle is in fluid communication with a second variable volume chamber similar to the first variable volume chamber. The fluid connections between the headspace of the vial and either the first or the second variable volume chamber includes a sample loop which is in operative connection with, or which may be connected, to an analytical instrument.
In operation of the exemplary embodiment the volumes of the first and second variable volume chambers are varied periodically and in a controlled synchronized manner. The volume of the first chamber is reduced as the volume of the second chamber is correspondingly increased. As this occurs vapor flows through the headspace of the vial, through the sample loop and into the chamber whose volume is increasing. Thereafter the flow is reversed such that the volume of the other chamber begins to increase while the volume of the other chamber decreases. As this occurs vapor flows in the opposite direction through the headspace and the sample loop. Because the volumes of the headspace, the sample loop and the first and second chambers remain constant, thermodynamic equilibrium is maintained in the headspace of the vial while the vapor phase of the sample is passed through the sample loop.
The vapor phase sample in the sample loop is analyzed through use of an analytical instrument. This may be accomplished for example, by switching a valve such that the material in the sample loop is passed to a gas chromatograph. Alternatively the sample loop may include sensors such as optical cells of spectrophotometers, semiconductor aroma sensors or other suitable sensors for analysis of the constituents in or the properties of the vapor phase of the substance.